The present invention relates to field effect semiconductor devices with high operation speed, which are preferably employed for circuits in which a super high frequency of 4 through 5 GHz or higher is employed, and methods for production thereof. Specifically, the present invention relates to an improvement applicable to field effect semiconductor devices provided with a compound semiconductor substrate, e.g. GaAs, GaAlAs, and with a Schottky gate of a silicide of a refractory metal, e.g. tungsten (W), molybdenum (Mo), tantalum (Ta), titanium (Ti) and the like. More specifically, this invention relates to configurations of such refractory metal silicide Schottky gates wherein the resistance is decreased, the gate length is decreased and the distance between the source and drain is decreased, and methods for production thereof.
A high operation speed is one of the most important requirements for field effect semiconductor devices which are employed for circuits in which a super high frequency of 4 through 5 GHz or higher is employed. Among the various parameters effective to improve the operation speed of field effect semiconductor devices, a higher mobility of carriers, e.g. electrons, is very important and essential. Employment of a compound semiconductor, e.g. GaAs or GaAlAs, is effective to satisfy this requirement. This is the reason why a compound semiconductor is preferably employed in the production of field effect semiconductor devices for circuits in which a super high frequency of 4 through 5 GHz or higher is employed.
Since it is rather difficult to produce an insulated gate on a compound semiconductor, a Schottky gate is employed. Since Aluminum (Al) has a sufficient conductivity and a sufficient barrier height between itself and a compound semiconductor and easily produces a fine pattern thereof, Al Schottky gates have historically been employed for field effect semiconductor devices of compound semiconductors.
However, it is observed that Al gates of compound semiconductor corrode under humid conditions. In order to overcome this drawback of corrosion, it was discovered that a refractory metal, e.g. W, Ti, Mo, Ta or the like, is an appropriate material for production of a stable and reliable Schottky gate of a compound semiconductor based field effect semiconductor device. More specifically, it was discovered that a treble layer comprising of a refractory metal, Platinum (Pt) and gold (Au) is useful from a realistic viewpoint, because a smaller resistance is readily realized without any minor problems which may unavoidably occur with a direct contact of Au and a refractory metal.
On the other hand, in order to satisfy some of the other requirements for a higher operation speed of field effect semiconductor devices, namely a shorter gate length and a shorter distance between the source and drain, it is required that impurity be implanted in the substrate employing a Schottky gate as a mask. However, this requirement can hardly be realized because a Schottky contact existing between a refractory metal and a compound semiconductor is converted to an ohmic contact when it is exposed to a temperature of 600.degree. C. or higher. In other words, a Schottky gate produced by depositing a refractory metal on a compound semiconductor substrate loses the potential barrier between itself and the compound semiconductor substrate and reduces the inverse breakdown voltage between itself and the compound semiconductor substrate when it is exposed to a temperature of 600.degree.C. or higher for the purpose of activating the impurities implanted in the substrate.
In order to overcome this difficulty, a Schottky gate of a refractory metal alloy, e.g. Ti-W, was developed. This refractory metal alloy Schottky gate can not maintain the barrier height either, when it is exposed to a temperature of 750.degree. C. or higher. Further, this material is inclined to react with GaAs and or to corrode during the production process, resultantly increasing the resistance thereof. Therefore, a refractory metal alloy is not necessarily a satisfactory material for production of a Schottky gate for a field effect semiconductor device of a compound semiconductor.
It was recently proposed in U.S. Ser. No. 334,923 to produce a Schottky gate with a silicide containing one or more refractory metals, e.g. W-silicide, Ti-W-silicide, Mo-silicide, Ta-silicide, et al., because this Schottky gate can be used as a mask for an ion implantation process followed by an annealing process conducted at an approximate temperature of 800.degree. C. When this material is employed for production of a Schottky gate, the gate length can be made approximately 1 micrometer and the distance between the source and drain can be made approximately 3 micrometers or less, which is the minimum dimension realized by the present technical level of the photolithography, resultantly realizing a field effect semiconductor device having a high operation speed.
It is well known that a shorter gate is effective to increase the operation speed of a field effect semiconductor device. Therefore, if the length of a Schottky gate of a silicide comprising one or more refractory metals could be shorter than the minimum dimension allowed for using photolithography, it would evidently increase the operation speed and realize a field effect semiconductor device having a higher operation speed. In addition, the Schottky gate of a silicide of a refractory metal has a drawback in that the resistance is relatively high and is on the order of l0.sup.-4 ohm-cm. Therefore, if this drawback is overcome, it would increase the operation speed of the field effect semiconductor device having a Schottky gate of a silicide or one of more refractory metals.